A report from the Wall Street Journal suggests that Google has been bypassing the privacy settings…
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According to Microsoft VP Dean Hachamovitch, after the Safari Google cookies snafu hit, the Internet Explorer team discovered that Google was "employing similar methods to get around the default privacy protections in IE and track IE users with cookies" too. Microsoft has found that Google bypasses the P3P Privacy Protection feature in IE to track users. Google is breaking the rules:

Google utilizes a nuance in the P3P specification that has the effect of bypassing user preferences about cookies. The P3P specification (in an attempt to leave room for future advances in privacy policies) states that browsers should ignore any undefined policies they encounter. Google sends a P3P policy that fails to inform the browser about Google's use of cookies and user information. Google's P3P policy is actually a statement that it is not a P3P policy. It's intended for humans to read even though P3P policies are designed for browsers to "read"

Basically, Google wrongfully bypasses the protection and enables its cookies to be allowed rather than blocked. Thus allowing Google to track the browsing habits of Internet Explorer users. In the mean time, Microsoft has made a Tracking Protection List to protect IE9 users from Google. The list can be found here. [IE Blog]